Laser-assisted device alteration using synchronized laser pulses

ABSTRACT

A pulsed-laser LADA system is provided, which utilizes temporal resolution to enhance spatial resolution. The system is capable of resolving CMOS pairs within the illumination spot using synchronization of laser pulses with the DUT clock. The system can be implemented using laser wavelength having photon energy above the silicon bandgap so as to perform single-photon LADA or wavelength having photon energy below the silicon bandgap so as to generate two-photon LADA. The timing of the laser pulses can be adjusted using two feedback loops tied to the clock signal of an ATE, or by adjusting the ATE&#39;s clock signal with reference to a fixed-pulse laser source.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation Application of U.S. patentapplication Ser. No. 13/896,262, filed on May 16, 2013, which is aContinuation-in-Part of U.S. patent application Ser. No. 13/228,369,filed on Sep. 8, 2011, which claims priority benefit from U.S.Provisional Application No. 61/381,023, filed on Sep. 8, 2010, thedisclosures of which are incorporated herein by reference in theirentirety. U.S. patent application Ser. No. 13/896,262 also claimspriority benefit from U.S. Provisional Application No. 61/648,042, filedon May 16, 2012, the disclosure of which is incorporated herein in itsentirety.

GOVERNMENT LICENSE RIGHTS

This invention was made with government support under USAF ContractFA8650-11-C-7104 awarded by IARPA. The government has certain rights inthe invention.

BACKGROUND

1. Field of Invention

This invention is in the field of laser-based defect localizationanalysis of integrated circuits (IC). More specifically, this inventionis about design debug and/or failure analysis of ICs using the laserassisted device alteration (LADA) technique.

2. Related Art

LADA (Laser-Assisted Device Alteration) is a technique that depends onthe ability of a continuous-wave (CW) laser to generate localizedphotocurrents in an integrated circuit through its backside and thuschange the pass/fail outcome of a test stimulus on a “sensitive”transistor, thereby localizing sensitive areas including design orprocess defects. The laser is used to temporarily alter the operatingcharacteristics of transistors on the device. The current spatialresolution using the 1064 nm continuous wave laser is 240 nm.

An explanation of the LADA techniques can be found in, for example,Critical Timing Analysis in Microprocessors Using Near-IR Laser AssistedDevice Alteration (LADA), Jeremy A. Rowlette and Travis M. Eiles,International Test Conference, IEEE Paper 10.4, pp. 264-273, 2003. Thatpaper described the possibility of using a CW laser of 1064 nm or 1340nm wavelength. It is explained that the 1340 nm would cause devicealteration via localized heating, while the 1064 nm would cause devicealteration via photocurrent generation. It is also noted that the 1064nm laser has a spatial resolution advantage. Therefore, the authorsrecommend the use of 1064 nm laser.

As shown in FIG. 1, conventional LADA uses a CW laser to induceelectron-hole pairs in the device under test from the backside. Theelectron-hole pairs so generated affect the timing of the nearbytransistor—thus facilitating critical path analysis. A DUT 110 iscoupled to a tester 115, e.g., a conventional Automated TestingEquipment (ATE), which is connected to computer 150. The ATE is used ina conventional manner to stimulate the DUT with test vectors and studythe response of the DUT to the test vectors. The response of the DUT tothe test vectors can be further investigated using the LADA. Forexample, if the DUT fails a certain test, LADA can be used toinvestigate whether the DUT can pass under certain conditions and, ifso, which device, i.e., transistor, was responsible for the failure.Conversely, when the DUT passes certain tests, LADA can be used toinvestigate under which conditions the DUT will fail these tests and, ifso, which device, i.e., transistor, was responsible for the failure.

The LADA system for FIG. 1 operates as follows. Tiltable mirrors 130 and135 and objective lens 140 are used to focus and scan a beam from CWlaser 120 onto the DUT 110. This allows the laser 120 to generate photocarriers in the silicon of the DUT without resulting in localizedheating of the device. The electron-hole pairs so generated affect thetiming of the nearby transistor, i.e., decreasing or increasingtransistor switching time. The tester is configured to place theoperating point of the device under test in a marginal state by applyinga recurrent test loop of selected voltage and frequency. The laserstimulation is then used to change the outcome of the tester's pass/failstatus. The beam's location at each point is correlated to the pass/failoutcome of the tester, so that when a change is detected, i.e., apreviously passing transistor is now failing or vice versa, thecoordinates of the laser beam at that time points to the location of the“borderline” transistor.

During the LADA analysis, the tester (ATE) is configured to place theoperating point of the device under test in a marginal state. The laserstimulation is used to change the outcome of the tester's pass/failstatus. The present state-of-the-art in laser assisted fault spatiallocalization is about 240 nm resolution. The limitation on furtherimprovement of the single photon LADA spatial resolution is imposed bythe laser light wavelength. As noted in the Rowlette paper, the spatialresolution is enhanced by using shorter wavelength. However, opticalabsorption of silicon at wavelength smaller than 1064 nm prevents theuse of shorter wavelengths, as it becomes the major obstacle fordelivering light to the transistor through the backside. Thus, whiledesign rules of modern devices shrink, the spatial resolution of theLADA system cannot be improved by the use of smaller wavelength laser.For example, at 22 nm design rule it is doubtful that conventional LADAwill be able to resolve among four neighboring transistors.

Optical beam induced current (OBIC) is another test and debug analysisin which laser beam is illuminating the DUT. However, unlike LADA, OBICis a static test, meaning no stimulus signal is applied to the DUT.Instead, the laser beam is used to induce current in the DUT, which isthen measured using low-noise, high-gain voltage or current amplifiers.OBIC has been used in a single-photon mode and in a two-photonabsorption mode, sometimes referred to as TOBIC or 2P-OBIC (two-photonoptical beam induced current).

Two-photon absorption (TPA) is the simultaneous absorption of twophotons of identical or different frequencies in order to excite amolecule from one state (usually the ground state) to a higher energyelectronic state. The wavelength is chosen such that the sum of thephoton energy of two photons arriving at the same time is equal to theenergy difference between the involved lower and upper states of themolecule. Two-photon absorption is a second-order process, severalorders of magnitude weaker than linear (single-photon) absorption. Itdiffers from linear absorption in that the strength of absorptiondepends on the square of the light intensity, thus it is a nonlinearoptical process.

SUMMARY

The following summary of the disclosure is included in order to providea basic understanding of some aspects and features of the invention.This summary is not an extensive overview of the invention and as suchit is not intended to particularly identify key or critical elements ofthe invention or to delineate the scope of the invention. Its solepurpose is to present some concepts of the invention in a simplifiedform as a prelude to the more detailed description that is presentedbelow.

Various embodiments disclosed enable higher spatial resolution of faultlocalization by utilizing time domain to enable enhanced spatialresolution. Disclosed embodiments utilize a pulsed laser with sufficientenergy in lieu of a continuous wave laser. The pulsed laser issynchronized to the device's clock, thus enabling improved spatialresolution. Various embodiments utilize a 1064 nm wavelength laser for asingle-photon LADA, or longer wavelengths so as to exploit thenon-linear two-photon absorption mechanism to induce LADA effects. Thislater technique is referred to herein as Two-Photon Laser AssistedDevice Alteration technique (2pLADA).

Disclosed embodiments enable higher resolution of fault localization byusing test vectors stimulating a DUT and at the same time use afemtosecond pulsed laser to scan an area of interest in the DUT andexamine the response of the DUT to the test vectors during the scan. Thelaser source is chosen such that the wavelength provides photon energybelow the band gap of silicon and it provides pulses of femtosecondpulse width. A clock signal is obtained from the ATE and is fed to theDUT and to the circuit controlling the pulsed laser. The timing of thepulses can be shifted relating to the ATE clock, so as to investigatethe pass/fail characteristics of various devices. Additionally, by usingproper synchronization of the laser pulses to the clock, spatialresolution is enhanced to enable resolving multiple devices, i.e.,transistors, within the laser beam.

In alternative embodiments, a fixed pulse laser system is used. Theclock of the fixed pulse laser is sent to the ATE and is used to derivethe test signals to the DUT. Moreover, in order to achieve the effect ofshifting the timing of the pulses relative to the test signal, the clocksignal of the fixed pulsed laser is shifted prior to sending it to theATE. Consequently, as the ATE generates the test signals based on theshifted laser clock, the test signal itself is shifted relative to thelaser pulses. By controlling the shift of the laser clock, the testsignals are shifted relative to the laser pulses to enable the LADAinvestigation.

Various embodiments provide a laser assisted device alteration (LADA)system operable in conjunction with an automated testing equipment (ATE)for testing integrated circuit device under test (DUT), comprising: acontroller receiving and analyzing test signals from the ATE; timingelectronics receiving a clock signal from the ATE, the timingelectronics including a first feedback loop generating a synch signalfor synchronization of laser pulses to the clock signal; a tunablepulsed laser source generating the laser pulses and having a tunablelaser cavity and having a second feedback loop to control the tunablelaser cavity so as to generate desired pulse rate of the laser pulses;optical arrangement receiving laser pulses from the tunable pulsed lasersource and directing the laser pulses onto desired location on the DUT;wherein the timing electronics is configured for timing the laser pulsesto arrive at transistors in the DUT at times synchronized to the clocktime to thereby alter the transistors response to test signals appliedto the DUT from the ATE, and wherein the controller is configured todetect the altered transistors response. The first feedback loop and/orthe second feedback loop may comprise phase-locked loop. The opticalarrangement may comprise a laser scanning microscope. The opticalarrangement may further comprise a solid immersion lens. The pulse rateof the laser pulses may be configured as a multiple of the clock signal.The multiple may be an integer larger than one or a fraction. The timingelectronics may be configured to delay or advance the laser pulses withrespect to the clock signal.

According to further embodiments, a laser assisted device alteration(LADA) system operable in conjunction with an automated testingequipment (ATE) for testing integrated circuit device under test (DUT)is provided, comprising: a controller receiving and analyzing testsignals from the ATE; a fixed pulsed laser source generating laserpulses and generate pulse rate signal indicative of pulse rate of thelaser pulses; timing electronics receiving the pulse rate signal andsending a clock signal to the ATE; optical arrangement receiving laserpulses from the fixed pulsed laser source and directing the laser pulsesonto desired location on the DUT; wherein the timing electronics isconfigured for adjusting the timing test signals output by the ATE toarrive at transistors in the DUT at times synchronized to the laserpulses, to thereby detect whether the laser pulses alter the transistorsresponse to the test signals applied to the DUT from the ATE, andwherein the controller is configured to detect the altered transistorsresponse.

The timing electronics may further comprise a variable phase circuitconfigured to vary the phase of the clock signal with respect to thepulse rate signal. The variable phase circuit may be configured toretard or advance the test signals with respect to the pulse ratesignal.

According to yet further embodiments, a method for testing an integrateddevice under test (DUT) that is coupled to an automated testingequipment (ATE) using laser assisted device alteration (LADA) isprovided, the method comprising: obtaining clock signal from the ATE andapplying the clock signal to the DUT; obtaining a test loop signal andapplying the test loop signal to the DUT; applying a first feedback loopto a pulsed laser source to generate laser pulses at repeatable rate;applying the clock signal to a second feedback loop to synchronize thelaser pulses to the clock signal; and directing the laser pulses ontodesired areas of the DUT. The first and or second feedback loops maycomprise a phase locked loop having an external reference signal. Theexternal reference signal may comprise the clock signal. The laserpulses may comprise picosecond to femtosecond laser pulses. The pulsedlaser source may be operated to generate laser pulses having wavelengthselected to generate single-photon laser assisted device alteration. Thepulsed laser source may be operated to generate laser pulses havingwavelength selected to generate two-photon laser assisted devicealteration. Directing the laser pulses may comprise scanning the laserpulses over a desired area of the DUT.

According to other embodiments, a method for testing an integrateddevice under test (DUT) that is coupled to an automated testingequipment (ATE) using laser assisted device alteration (LADA) isprovided, the method comprising: using a fixed-pulse laser source togenerate laser pulses at a designated pulse rate; obtaining pulse ratesignal from the fixed-pulse laser source and generating clock signaltherefrom; applying the clock signal to the ATE and generating a testloop signal by the ATE and applying the test loop signal to the DUT; anddirecting the laser pulses onto desired areas of the DUT. The method mayfurther comprise varying the phase of the clock signal with respect tothe pulse rate signal.

According to further embodiments, a method for measuring carrierlifetime in a semiconductor device under test (DUT) using laser assisteddevice alteration (LADA) technique is provided, comprising: repeatedlyapplying electrical test signal to the DUT; using a laser beam toilluminate a sensitive transistor in said DUT, so as to obtain anoptimum LADA signal, thereby obtaining spatial coordinates of thesensitive transistor; using a pulsed laser source to generate laserpulses and illuminating the sensitive transistor using the spatialcoordinates to direct the laser pulses; varying timing of the laserpulses and recording strength of the LADA signal for each timingposition until the strength of the LADA signal has reached zero; anddetermining the carrier lifetime by calculating a time period taken forthe LADA signal to change from maximum to minimum signal. The laser beamfor determining the spatial coordinates may be obtained from acontinuous-wave (CW) laser source. The step of calculating the timeperiod may comprise plotting the LADA signal strength vs. time. Themethod may further comprise a step of directing the laser pulses at thespatial coordinates and varying the timing until optimum LADA signalstrength is obtained, so as to determine the pulse timing for maximumlaser pulse overlap with test signal overlap.

BRIEF DESCRIPTION OF DRAWINGS

Other aspects and features of the invention would be apparent from thedetailed description, which is made with reference to the followingdrawings. It should be appreciated that the detailed description and thedrawings provides various non-limiting examples of various embodimentsof the invention, which is defined by the appended claims.

The accompanying drawings, which are incorporated in and constitute apart of this specification, exemplify the embodiments of the presentinvention and, together with the description, serve to explain andillustrate principles of the invention. The drawings are intended toillustrate major features of the exemplary embodiments in a diagrammaticmanner. The drawings are not intended to depict every feature of actualembodiments nor relative dimensions of the depicted elements, and arenot drawn to scale.

FIG. 1 illustrates a prior art CW LADA system.

FIG. 2 illustrates an embodiment of pulsed-laser LADA system.

FIG. 2A illustrates an embodiment of the two feedback loops.

FIG. 2B illustrates an embodiment using a fixed-pulsed laser source togenerate clock signal.

FIG. 3 illustrates an embodiment to achieve a synchronization scheme.

FIGS. 4A-4C illustrate how pulsed laser LADA helps individually identifyand isolate both the P- and NMOS transistor locations in closeproximity.

FIGS. 5A-5D illustrate enhanced spatial resolution as a consequence ofaccurate pulse placement capabilities.

FIG. 6 illustrates an embodiment of laser repetition rate lockingscheme.

FIGS. 7A-7C illustrate steps taken to measure minority carrier lifetimeusing LADA, according to one embodiment.

DETAILED DESCRIPTION

FIG. 2 illustrates an embodiment which uses a pulsed laser source withsufficient energy in lieu of a continuous wave laser. This embodimentconcerns applying photon absorption to precisely inject carriers into anIC for the purposes of fault localization using the LADA technique andcan be used for IC characterization and finding ways to improve thedesign. The technique is based on photons arriving at a focus in thetransistor so that the delivered photon energy is greater than what isneeded for electron hole pair creation (>1.1 eV in silicon, for example;other ICs such as GaAs, SiGe, InP, etc, have different bandgapenergies). Photon stimulation in this embodiment requires excitationwith laser pulses in the range of nanoseconds to femtoseconds. Thesignal is localized to the focal spot of the laser, providing animmediate improvement in fault localization. The effective volume inwhich the electron-hole pair generation occurs seems to be reduced dueto the synchronization. The embodiments use sophisticated timingelectronics to precisely control the timing of the laser pulse withrespect to the transition of an edge of the tester clock, e.g., ATEclock. This type of control allows to finely vary the delay or advanceof signals propagating through the transistor of interest for LADA.

FIG. 2 illustrates an embodiment of the invention, wherein a DUT 210 iscoupled to an ATE 215, as in the prior art. However, in the embodimentof FIG. 2, nanosecond to femtosecond laser pulses are generated bypulsed laser source 225, which are then focused onto the DUT 210 usingtiltable mirrors 230 and 235 and objective lens 240. For 2pLADA thelaser source 225 provides a pulsed laser beam of wavelength that islonger than the silicon bandgap, i.e., longer than 1107 nm. In oneembodiment wavelength of 1550 nm is used, while in another 1340 nm or1250 nm are used. On the other hand, the same arrangement can also beused for single-photon LADA, in which case the laser source can providepulsed beam of wavelength such as 1064 nm. In this embodiment, thetiltable mirrors 230 and 235 are implemented as a laser scanningmicroscope (LSM). Also, in some embodiments a solid immersion lens (SIL)is used as part of the objective lens arrangement 240.

In conventional LADA systems the laser is always on; however, accordingto embodiments of the invention, very short pulses are used. Therefore,it is important that the device transition occurs when the laser pulsearrives at the device. To achieve that, a trigger signal 245 is obtainedfrom the ATE and input to timing electronics 260, which controls thepulsed laser 225 to synchronize the laser pulses with the test signalsof the ATE.

Using the system shown in FIG. 2, first the tester (ATE) 215 is operatedto apply a set of test vectors to determine the marginal settings of theDUT 210. That is, the voltage and frequency of the test vectors arevaried to determine the point where the DUT is just about to fail, orhas just failed the test, e.g., the DUT is failing the test 50% of thetimes of a test loop. This is the DUT's pass/fail boundary condition.The voltage and frequency settings are then used to generate arepetitive test signal to repeatedly stimulate the DUT at its pass/failboundary condition.

As the DUT is stimulated at the boundary condition, a sync signal 245 issent from the tester 215 to the timing electronics 260. The timingelectronics 260 controls the laser source 225 to obtain laser pulses ofpicosecond to femtosecond pulse width and of wavelength higher thansilicon band gap for 2pLADA or shorter for single-photon LADA. Ingeneral, for 2pLADA the wavelength is about 1250 nm to 1550 nm and thepulse width is about 100 fs. For single-photon LADA the wavelength maybe 1064 nm and the pulse width is about 100 fs. The laser pulses arescanned over an area of interest in the DUT 240 to thereby increase ordecrease the DUT's switching time and push the DUT beyond the boundarycondition. That is, if the voltage/frequency of the test vector are setsuch that the DUT is just about to fail, the laser pulses are timed tocause the DUT to fail. Conversely, if the voltage/frequency of the testvector are set such that the DUT is just failing, the laser pulses aretimed to cause the DUT to pass the test. During this time the output ofthe DUT is monitored to determine location of the failure. That is, atthe moment in time where the output signal from the DUT indicates afailure (where without the laser beam the DUT would pass), the locationof the beam over the DUT is determined, to thereby determine thelocation of the transistor causing the failure. Conversely, at themoment in time where the output signal from the DUT indicates a pass(where without the laser beam the DUT would fail), the location of thebeam over the DUT is determined, to thereby determine the location ofthe transistor previously causing the failure and now passing.

It should be appreciated that since a sync signal is obtained from thetester, the timing of the laser pulses can be varied so as to vary theamount of the photo-generation (single-photon or two-photon) effect onthe transistor. That is, the timing of the laser pulses can be varied soas to increase or decrease the amount of increase or decrease the DUT'sswitching time. This ability can assist in determining the severity ofthe fault, in addition to its location.

Embodiments of the invention also use timing electronics to preciselycontrol the timing of the laser pulse with respect to the transition ofan edge of the tester (e.g., ATE) clock. This type of control allows tofinely vary the delay or advance of signals propagating through thetransistor of interest. In one example, two phase locked loops (PLL) areused to precisely control the pulsed laser, as illustrated in FIG. 2A.In FIG. 2A the ATE 215 provides a clock signal, Clk, and a test loopsignal, Test Loop. Both clock signal and test loop signal are input tothe DUT and are tapped and sent to the timing electronics 260, whichforms the first PLL. The laser source 225 includes a second PLL.

That is, the PLL of the laser source 225 ensures that the pulsefrequency of the laser pulses is stable and accurate to the desiredfrequency, e.g., 100 MHz. Conversely, the first PLL of the timingelectronics provides synchronization of the frequency of the second PLLto the clock signal of the ATE. Notably, in this context,synchronization does not necessarily mean that the laser pulses and theclock pulses are concurrent, but rather that they are synchronized overa test loop period. So, for example, the timing of the laser pulses maybe shifted, such that the pulses appeal at the middle of the clocksignal for every clock pulse, as illustrated by pulse train 227, or atthe end of each clock pulse, as shown by pulse train 229, etc. That is,the laser pulses may be delayed or advanced with respect to the clocksignal of the ATE, but remain synchronized to the clock signal of theATE.

On the other hand, as will be elaborated below, the frequency of thelaser pulses may be a multiple of the ATE clock signal. For example,laser pulse train 223 has a multiplier of 7, such that seven laserpulses are generated for every one clock pulse of the ATE. Using amultiplier larger than 1, one can probe whether the failure is at therising edge, trailing edge, etc. Also, it is not necessary to providedelay or shift of the pulses, since the plurality of laser pulses foreach clock pulse serve the advance/delay function. Conversely, it isalso possible to have a multiplier smaller than 1. For example, in thepulse train 224 the multiplier is ½, such that a laser pulse arrivesonly every other clock signal. Such an arrangement can be used tovalidate that the failure is indeed due to the laser pulses, since if itis due to the laser pulses, then the device would fails about 50% of thetime.

FIG. 3 illustrates an embodiment to achieve a synchronization scheme.The output pulses from a pulsed laser source (1) of nano-to-femtosecondtemporal duration can be synchronized to the clock cycle of anintegrated circuit (IC) (2) through an intermediary phase-locked loop(PLL) circuit (3). In this configuration, the PPL circuit accepts theclock cycle frequency of the IC and locks it to an internal crystaloscillator of the same frequency. In this embodiment, the clock andcrystal oscillator frequencies are fixed at 100 MHz. The IC clock signalcan be generated by the ATE (not shown). This enables a 1:1 opticalpulse to transistor-switching-event synchronization ratio. Under theseconditions, practically speaking, these values can be fixed anywhere inthe range from 1 kHz to 10 GHz, before the efficiency of the photonabsorption rate becomes severally diminished.

It is noted here that optical sources faster than 1 GHz are notdesirable for nonlinear studies such as with 2pLADA, since in generalthe peak optical power contained with each pulse is inverselyproportional to the repetition rate. Therefore, high repetition ratesequal low peak optical powers which results in ineffective, if any,two-photon absorption. For 1:1 synchronization ratio single-photonstudies at 1064 nm on the other hand, multi-GHz optical sources may bebeneficial since the optoelectronic interaction scales linearly withincident optical power. In addition, it should be noted that theefficiency of two-photon absorption is directly proportional on theincident pulse duration, where femtosecond optical pulses facilitatehigher peak optical powers than a picosecond or nanosecond alternativeand therefore providing improved nonlinear absorption. As a result, itis desirable for nonlinear studies to utilize ultrafast (i.e. picosecondor femtosecond) optical pulses. On the other hand, for single-photonstudies, the pulse duration is not a limiting parameter with regard toabsorption rate and so does not restrict performance. If anything, itenables an additional probing parameter (e.g. for measuring opticalpulse interaction periods vs. photoelectric device stimulation).Furthermore, the silicon absorption coefficient is greater forsingle-photon wavelengths <1130 nm compared totwo-photon-absorption-tuned wavelengths (i.e. □□>1250 nm).

In order to maintain the efficiency, one can modify the synchronizationscheme in order to match integer multiples of incident optical pulses totransistor switching events (or device clock frequencies). For this tosucceed, the laser source must be designed to produce a higher than 1GHz repetition rate and have a scalable pulse picker module in place,post pulse optimization, in order to amend the synchronization ratios.For example, instead of matching each incident optical pulse to everytransistor switching event, one could match every second pulse to everysubsequent switching event; thus, creating a 2:1 synchronization ratio.Practically, this would translate into the use of a 200 MHz repetitionrate laser and a 100 MHz device frequency, or a 1 GHz repetition ratelaser and a 500 MHz device frequency, and so on. Alternatively, onecould tune the ratio to be 3:1 or 4:1, etc., so long as the ratiocorresponds to an optoelectronic overlap with the clock pulse insynchronization with the test loop signal. Under this synchronizationscheme, the efficiency of photon absorption is not decreased; however,the rate at which absorption takes place does, generating a photonsignal intensity that is negatively scaled as per the synchronizationratio. It must be noted here that this may not be a limiting factor forlaser-induced studies of integrated-circuits. For each device undertest, there is a requirement to perform a photon scalability calibrationin order to determine the maximum synchronization ratio allowed.Furthermore, by integrating a tunable optical source (i.e. 1000-1600 nmoutput wavelength) into such a system, one can interchange betweensingle-photon and two-photon absorption regimes since two-photonabsorption begins to dominate over single-photon absorption atwavelengths larger than about 1200 nm in silicon.

Once these frequencies (i.e. the clock and crystal oscillatorfrequencies) are locked together, the PLL circuit output signal is thensent through a 100 MHz (or clock frequency) electronic filter to thepulsed laser to serve as its input stimulus. The benefit here is thatthe PLL circuit has full control over the phase of its output signal.Therefore, one can control the repetition rate, and hence the pulsearrival time, of the laser's optical output. This can be verified bycomparing the output clock frequency from the IC to the trigger outputfrom the pulsed source on an oscilloscope (9). In this example the PLLcircuit can electronically accommodate phase delays of about 600 fs;however, due to the board's electrical jitter, a minimum phase delay isset to about 20 ps. The system's electrical jitter is directlyproportional to the accuracy at which the optical pulses can bepositioned relative to the switching time of the individual transistorsunder evaluation. Therefore, since the system's electrical jitter is 20ps, the optical placement accuracy is also 20 ps—there is a one-to-onecorrespondence. This is a crucial parameter since it can negate thetiming benefits to be gleaned if the electronic placement error islarger, for instance, than the 2pLADA femtosecond pulse duration.Femtosecond optical pulses increase the local energy density as requiredfor efficient two-photon absorption; however, when the electrical jittereclipses this isolated carrier generation timescale then the jitter canlimit the subsequent signal generation and temporal precision of the nowtime-resolved data.

The laser pulses are then coupled into a laser scanning microscope (LSM)(4) where they can be accurately distributed across a particularlocation on the IC. The LSM is controlled using a computer (6) whichhosts a graphical user interface as well as a custom digital signalprocessor (DSP) suite. In disclosed embodiments, this applications suitegives the end user, through a preconfigured DSP circuit (7), the abilityto directly communicate with the PLL circuit which in turn provides fullcontrol over the laser pulse's arrival time at the device, e.g., bydelaying or advancing the pulses.

With regard to the device 2, it can be electrically stimulated toproduce preconditioned LADA pass/fail values through the use of a customapplications interface (5). This board compares a real-time acquisitionvalue from a counter, latch and shift register arrangement against aloaded reference value which was inserted beforehand by selecting areset switch. Fine control over the real-time loaded counter value canbe controlled through an analog fine delay potentiometer on theapplications interface board which alters the timing of the latch enablefunction on the IC. This configuration allows the user to conditionpredominantly passing, failing or balanced comparator output values.These pass/fail output values are then sent to a data conditioningcircuit (in this example, a field-programmable gate array FPGA 8) whichis programmed to accept a real-time digital pass/fail stimulus,conditioned to scale between failing values from 0-100%, and delivers anaveraged (about 40 us period) digital output, again scaled between0-100% fail, for enhanced visualization and biasing of the resultingpass/fail levels in the graphical user interface. The data conditioningcircuit can also be used in conjunction with the applications interfaceboard in order to calculate the magnitude of laser-induced timing delaysby calibrating the application board's output delay voltage.

In the embodiments described above, a tunable pulsed laser source isused, and the pulse frequency is tuned so as to synchronize to the ATEclock. While these embodiments are operable, a tuned pulsed laser sourceis rather expensive and requires the phase locked loop described above.FIG. 2B illustrates another embodiment, which enables LADA testing usinga simpler fixed pulsed laser 255. For example, a mode-locked lasersource can be used. Mode-locking is a technique in optics by which alaser can be made to produce pulses of light of extremely shortduration, on the order of picoseconds or femtoseconds. The laser pulsesare used as the clock, which is fed to the timing electronics 265.Conventional ATE's have clock input port and are programmable to use theinput clock in order to generate the clock, Clk, and the test loopsignals for the DUT. Therefore, in one example, the clock signal fromthe timing electronics 265 is input to the ATE, and the ATE isprogrammed to use that input to generate the clock and test loop signal.

However, as noted above, to obtain the most benefit of the pulsed laserLADA, it is desirable to adjust the pulses such that the laser pulsesarrive at the transistor at different times during the clock cycle,e.g., leading edge, middle, trailing edge, etc. In the embodiments ofFIGS. 2, 2A and 3, this was done by advancing or retarding the laserpulses. However, in the embodiment of FIG. 2B the laser pulses are fixedand cannot be changed, so retarding or advancing the laser pulses is nota viable option. Therefore, according to one embodiment, the ATE isprogrammed to retard or advance its clock signal in synchronization withthe clock signal received from the timing electronics 265. In thismanner, the timing of the arrival of the laser pulses at the transistorcan be tuned to leading edge, trailing edge, etc., of the ATE clocksignal.

On the other hand, since in general the ATE and the LADA testers aremade by different manufacturers, and the actual testing is performed byan engineer from yet a third company, it would be beneficial to simplifythe operations of the testing engineer and offload the retardation oradvancement of the signal from the ATE. This is done by using the phaseshifter 275, illustrated in the embodiment of FIG. 2B. That is, usingthe phase shifter 275, the clock signal output from the timingelectronics 265 can be advanced or retarded with respect to the laserpulses. The resulting modified signal is then sent to the ATE as theinput clock signal. Consequently, when the ATE output its clock and testloop signals, they are both can be shifted or retarded with respect tothe laser pulses.

EXAMPLES

Constructing a pulsed LADA system with a pulsed optical source allowsfor novel aspects of the operating device to be evaluated and measured.When traditional single-photon or alternative two-photon LADA utilize aCW laser, the optical radiation is constantly interacting with theindividual transistors with a potentially damaging level ofinvasiveness. A pulsed LADA methodology, on the other hand, allows theswitching characteristics of the individual transistors to be mapped outin as much as two-physical dimensions. Extended pulsed LADA concepts arediscussed in detail below.

Under conventional CW LADA stimulation, device theory and practiceinforms us that the magnitude of laser-induced device perturbations froma p-type metal-oxide-semiconductor (PMOS) transistor dominates over itsn-type (NMOS) neighbor. Since the diameter of the laser beam would coverboth the p-type and its neighboring n-type transistors, the resultingspatial resolution is insufficient to distinguish the failingtransistor. On the other hand, using the disclosed embodiments in apulsed arrangement, temporal resolution is used to achieve higherspatial resolution, even when a larger wavelength laser is used. Thatis, since the incident pulses are tuned to the exact temporal switchinginterval of the transistors under investigation, and also because thepeak power contained in each pulse is significantly higher than in CWmode, it is possible to individually identify and isolate both the P-and NMOS transistor locations in close proximity. This is impossibleunder CW excitation and therefore creates a new experimental avenue forsemiconductor device design debug and characterization to explore, evenat increasingly small design rules. This addresses a growing concernwithin the semiconductor device failure analysis community whereoptically-induced transistor recognition and operating characteristicsare of critical importance as the latest technology nodes are scaledtoward lower nanometer geometries. Consequently, synchronized pulsedLADA offers more value than its CW counterpart.

A schematic example of such improvements is presented in FIGS. 4A-4C. Incontinuous-wave mode, the PMOS signal generally dominates, resulting ina generalized spatial distribution of a single signal, as illustrated inFIG. 4A. It is incredibly difficult here to discern individual physicaltransistor distributions and/or match these LADA manifestations tocomputer-aided design (CAD) layouts. Theoretically, each transistorshould generate its own LADA signal—no matter the magnitude of thelaser-induced effect, as illustrated in FIG. 4B. These would perfectlytrace out the physical location of the individual transistors allowingfor rapid physical and/or optoelectronic recognition. This can bereplicated in the pulsed domain using the embodiments discussed above.That is, the laser pulses are timed and synchronized to the test signal,such that they arrive at the P- and NMOS transistor locations accordingto the user's selection. The pulses can be timed to the switching of thePMOS transistors to test them, or to the switching of the NMOS to testthe NMOS transistors, as illustrated in FIG. 4C. Therefore, uniquetransistor switching evaluations as well as CAD-enhanced physicalmapping/recognition of LADA signals can be performed, regardless of thespatial laser coverage.

Furthermore, an additional benefit of the increased peak power fromultrafast pulses (aside from the ability to generate LADA signals moreefficiently, i.e., acquiring fewer image averages) is the generalizedincrease/decrease (depending on whether you perturb a P- or N-MOStransistors) in laser-induced critical timing path perturbations andtherefore improved LADA signal collection. Larger incident optical powerincreases the number of photo-injected carriers within the silicon,which in turn enhances the probability of stimulating optoelectronicfluctuations within the device structures. This leads to superior LADAsignal responses which can be more readily measured with a reduced levelof invasiveness—pulsed optical sources are actually off for longer thanthey are on, limiting the opportunity for thermal build up and damage.For example, an ultrafast laser with a pulse duration of 10 ps and arepetition rate of 100 MHz is off for a duration of 10 ns—creating aratio of 1:1000 (ON:OFF)—thereby providing ample cooling off period.However, it should be noted that it is ultimately the power ratio thatcauses the heating. For example, a single optical pulse containing 1 kJof incident optical energy would satisfy the above criteria however italso contains enough energy to potentially permanently damage the devicethrough some other thermal or non-thermal optoelectronic mechanisms.

Also, with the facility to noninvasively inject significant levels ofoptical power into a specific transistor comes the opportunity todisturb previously overlooked transistor locations. Naturally, a greaterlevel of photo-carrier generation within the vicinity of a given regionof interest (which is populated with transistors of varying sensitivity)increases the probability of visualizing a wider range of LADAmanifestation sites. These activation regions can be stimulated withapproximately 10-100 uA of laser-induced photocurrent; however, with anultrafast laser pulse boasting peak optical powers approaching 10-100 kWit may be possible to inject 10-100 mA of photocurrent (whilemaintaining a safe level of invasiveness) within the device which may beenough to perturb ‘healthy’ transistors.

Efficient two-photon absorption can be obtained in silicon with a focallaser power density of more than 10 MW/cm2; however, single-photonvalues will be approximately 106 times smaller due to its relativeabsorption cross-section. The level of incident optical power (or localpower density) required for efficient, non-invasive photo-carrierinjection will decrease as the spatial geometries of the transistorsunder evaluation also decrease. Also the generation of two-photonabsorption is not dependent on a specific power densitythreshold—two-photon absorption is an instantaneous, quantummechanically defined, nonlinear process which is sensitive to theimaginary part of the third-order nonlinear susceptibility (i.e. itexhibits an intensity-squared dependence, not a power densitydependence).

Even though a two-photon wavelength of 1250 nm generates effectively 625nm inside the silicon (where the absorption cross-section is higher than1064 nm), the intensity-dependent nature of the absorption processreduces its overall relative rate of absorption. Two-photon absorptionis directly proportional to the square of the incident opticalintensity. In addition, silicon doping levels also contribute to thisdiscussion—i.e. increased or decreased doping concentrations affect thelevel of absorbance as a function of wavelength. Thissingle-photon-biased opportunity however enables another novellaser-probing and device characterization platform for enhanced criticaltiming analysis of racing/switching signal levels within transistors. CWLADA cannot offer this type of interrogation due to limitations frominvasiveness (i.e. the laser—is always on) and limited power deliverycapabilities. Time-resolved pulsed probing, on the other hand, may allowfailure analysts to investigate, for the first time, transistorswitching physics on healthy, design-defined nodes as well as thesubsequent down-chain device performance/interactions also. In order toeffectively implement this type of device characterization, anunderstanding of the level of incident optical power required plays animportant role. Perturbing ‘healthy’ transistors requires high peakpowers while facilitating a minimal level of invasiveness. With thatsaid, optimization of the temporal duration of the incident opticalpulses is required. Clearly, picosecond pulse durations at 1064 nmprovide significant levels of incident optical power (hence,photo-carrier generation) at the transistor level since, for example, a10 ps laser pulse at a repetition rate of 100 MHz and an average powerof 4 mW generates a peak power of 4 W; however, this may be limited ifthe laser repetition rate is matched to a faster than 1 GHz clockfrequency from a device under test. Increasing repetition rates resultsin decreasing peak powers. Therefore, a more suitable alternative is theuse of a femtosecond laser source. The laser repetition rate could bescaled in accordance with the device operation frequency while providingenhanced levels of peak optical power since the pulse duration hasreduced by a factor of 1000—increasing the peak power by the samemagnitude (4 kW in the example given above). An additional benefit ofthe femtosecond pulse duration would be the improvements in temporalcharacterization; however, this is restricted by the magnitude of thesynchronization scheme's electrical jitter—as described previously.Finally, femtosecond laser pulses provide a reduced level of opticalinvasiveness compared to picosecond or nano-second pulses, minimizingthe potential for laser-induced damage to the device.

In addition, a pulsed LADA system demonstrates enhanced spatialresolution as a consequence of accurate pulse placement capabilities.Again, when in CW mode, the laser continuously stimulates a specificregion of interest while inferring LADA information in real-time. Thisresults in a spatially averaged two-dimensional LADA image since thereis no discrimination between the highly ordered sequences of circuitfunctionality (i.e. propagating signal paths vs. time) where acollective distribution is acquired with a PMOS dominated bias. Inpulsed mode however, one can discern between these propagating speedpaths with an accuracy of about 20 ps, allowing for highly confined LADAsignal manifestations which have an enhanced lateral resolution sincethey individually and temporally address spatially separated neighboringtransistors that are not configured to switch until later in the deviceoperational cycle. This enhances the LADA isolation resolution as wellas the physical LADA resolution.

A schematic example is presented in FIGS. 5A-5D. In continuous-wavemode, since the spatial distribution of LADA signals is time-averaged,the resulting two-dimensional LADA map has a generalized optoelectronicstructure which may be biased according to the individual transistors'LADA signal strengths (since PMOS generally dominates over NMOS). Theseimages, illustrated in FIG. 5A, suffer from poor spatial resolution andlimited CAD overlay capabilities. In pulsed mode however, the LADA imagecan be enhanced and its spatial resolution improved due to thetime-resolved nature of the acquisition process. By individuallyaddressing each transistor as a function of both space and time (andwith sufficient incident optical power so as to remove any PMOS biasingeffects), neighboring transistor influence is effectively removed fromperturbing the LADA acquisition since there is now a tester-driven,transistor-dependent sequence of events controlling the deviceoperation. Each transistor is configured to switch in a systematic,time-dependent order, allowing for the incident optical pulse todirectly manipulate and measure each transistor in two physicaldimensions (i.e. X and Y) as well as time. As a result, the spatialresolution of the acquired LADA signal is improved and thereforeadditional, previously unobtainable, device data may be extracted, asillustrated in the sequence of FIG. 5B-5D, showing images taken atdifferent times, each with its temporal and spatial separation.

Aside from gleaning only LADA-specific data from this technique, it isalso possible to determine additional optoelectronic phenomena as well.One example would be the measurement of laser-induced carrier lifetimes.Currently, the carrier lifetime inside specific device locations istremendously difficult to quantify since it depends on a number ofdifferent optoelectronic parameters, such as material composition,dimensions, geometries and electric-field magnitudes and directions,etc. With pulsed LADA however, one may be able to directly measure thiselectronic timescale through a pseudo pump-probe type technique wherethe creation of a transistor-specific LADA event is linked to thearrival time of a laser pulse. The measured carrier lifetime may requirethe consideration (i.e. subtraction) of the system's electronic responsetime for a more accurate representation.

When using single-photon LADA, i.e., laser pulses of wavelength 1064 nm,the magnitude of the measured LADA effect is directly proportional tothe magnitude of the laser-induced photo-current (this is when usinglinear absorption, as the LADA signal would respond quadratically with atwo-photon technique). According to one embodiment, the LADA signal ismapped out as a function of laser pulse arrival time. Then it would bepossible to extract the carrier lifetime, since the lifetime willdictate the magnitude of the resulting LADA signal.

A process for accomplishing this according to one embodiment would be asfollows. First, a laser beam (e.g., a CW laser beam of wavelength 1064nm) is parked to illuminate a sensitive transistor, so as to obtain anoptimum LADA signal. This is illustrated in FIG. 7A. The spatialcoordinates of the laser beam at the optimum LADA signal indicates theproper spatial coordinates of the transistor. Then the CW laser sourceis disabled and the pulsed laser source is activated and the laserpulses are directed to the same spatial coordinates obtained from the CWlaser. The laser pulse timing is adjusted to obtain and measure theoptimum LADA signal, so as to obtain the proper temporal overlap withthe tester (e.g., ATE) pulses arriving at the transistor. This isillustrated in FIG. 7B. At this time, the optimized the spatial overlapof the laser spot with the transistor and the temporal overlap of thelaser pulses with the test signal have been achieved. Then, the laserpulses arrival time can be adjusted to measure the carrier lifetime.Specifically, the timing of the laser pulses is then varied and the LADAsignal strength is recorded for each timing position (e.g., retarded oradvanced amount), until the LADA signal has reached zero. The resultingLADA signal strength vs. time response is the plotted, as illustrated inFIG. 7C. The time taken for the LADA signal to change from maximum tominimum signal (or vice versa) corresponds to the measured laser-inducedcarrier lifetime. The above process is performed while repeatedlyapplying electrical test signal to the DUT.

Laser Source

Multi-GHz repetition rate laser sources are readily available and areconstructed through carefully consideration of their resonator cavitylength—i.e. the shorter the oscillating cavity, the higher therepetition rate. Control over the cavity length can be manipulated andlocked through the inclusion of a piezo-electric actuator located on theopposite side of an intra-cavity resonator mirror. This is the industrystandard technique for repetition rate locking; however, the electronicmixer circuit required to facilitate such a scheme may differ in designand implementation. To properly incorporate the tuned-pulsed lasersource into the LADA tester as described in the above embodiments, twofeedback loops are required; one to control the repetition rate of thelaser pulses and one to synchronize the timing of the pulses to the DUTclock. The first feedback loop, which controls the repetition rate,includes a mixer that compares the free-running repetition ratefrequency of the laser to an input clock stimulus in order to produce ahigh-voltage driven difference signal. The difference signal is input tothe piezo-electric transducer to adjust the resonator cavity length,which then adjusts the desired length so that the pulse rate matches theclock input supplied. An example of such a setup is illustrated in FIG.6. In addition to the circuit described in FIG. 6, a secondarystabilization scheme can be included to continually monitor and correctthe output voltage from the proportional-integral amplifier. Thisensures that the high voltage amplifier is consistently given thecorrect input voltage so to provide repetition rate locking stability ofa greater period of time—i.e. several days compared to tens of minutes.

It should be understood that processes and techniques described hereinare not inherently related to any particular apparatus and may beimplemented by any suitable combination of components. Further, varioustypes of general purpose devices may be used in accordance with theteachings described herein. It may also prove advantageous to constructspecialized apparatus to perform the method steps described herein.

While this invention has been discussed in terms of exemplaryembodiments of specific materials, and specific steps, it should beunderstood by those skilled in the art that variations of these specificexamples may be made and/or used and that such structures and methodswill follow from the understanding imparted by the practices describedand illustrated as well as the discussions of operations as tofacilitate modifications that may be made without departing from thescope of the invention defined by the appended claims.

The invention claimed is:
 1. A method for altering a switching time of adevice under test (DUT), the method comprising: determining marginalsettings of the DUT; applying electrical test signals to the DUT using atester to apply repetitive test signals based on the marginal settingsto repeatedly stimulate the DUT at a pass/fail boundary; with a lasersource, generating laser beam pulses having sufficient energy togenerate electron-hole pairs within the DUT; scanning the laser beampulses onto an area of interest in the DUT; controlling the timing ofthe laser source according to a clock of the tester; and whereinincident photon energy of the laser beam pulses is set to be less than abandgap of material forming a device in the DUT.
 2. The method of claim1, wherein the laser beam pulses are of wavelength longer than 1107 nm.3. The method of claim 1, wherein the laser beam pulses are of about 100femtoseconds pulse width.
 4. The method of claim 1, wherein controllingthe timing comprises controlling the timing of the laser pulses so as toincrease or decrease switching time of the DUT.
 5. The method of claim1, wherein controlling the timing comprises controlling the timing ofthe laser pulses with respect to a transition edge of the tester clock.6. The method of claim 1, further comprising varying the timing of thelaser beam pulses to determine severity of a fault in the DUT.
 7. Themethod of claim 1, wherein: the determining marginal settings comprisesselecting at least one voltage and at least one frequency; and therepetitive test signals are of the selected at least one voltage and theselected at least one frequency.
 8. A method for testing a device undertest (DUT), the method comprising: applying a first set of electricaltest signals from a tester to the DUT, so as to determine marginalsettings for the DUT; setting voltage and frequency of a second set ofelectrical test signals of the tester according to the marginal settingsfor the DUT; generating electrical DUT output signals by repeatedlyapplying the second set of electrical test signals from the tester tothe DUT; during the repeatedly applying, using a laser source togenerate laser pulses of wavelength higher than silicon band gap;controlling timing of the laser pulses according to a clock signal fromthe tester; applying the laser pulses to an area of interest in the DUTto thereby generate electron-hole pairs within the DUT; and monitoringthe electrical DUT output signals to determine location of a failure. 9.The method of claim 8, wherein monitoring the DUT output comprises ateach point in time monitoring a pass-fail of the DUT and correlating thepass-fail result to the location the laser pulses illuminate at thatpoint in time.
 10. The method of claim 8, wherein the determining themarginal settings comprises determining the voltage and frequency of atest signal such that the DUT is just about to fail the test, or hasjust failed the test.
 11. The method of claim 8, wherein incident photonenergy of the laser pulses is set to exactly equal or be greater thanhalf the DUT material bandgap.
 12. The method of claim 8, wherein thelaser pulses are of wavelength longer than 1107 nm.
 13. The method ofclaim 8, wherein the laser pulses are of about 100 femtoseconds pulsewidth.
 14. The method of claim 8, further comprising varying the timingof the laser pulses to determine severity of a fault in the DUT.